I. Field of the Invention
This invention relates generally to electronic sensors for monitoring both temperature change and sound using a single transducer, and more particularly to the construction of such a sensor that more readily allows the temperature signal to be separated from the sound signal using conventional signal processing techniques, e.g., filtering, and which provides a more robust output due to sound vibrations.
II. Discussion of the Prior Art
In the U.S. Pat. No. 5,311,875, which is hereby incorporated by reference, there is described a system for electronically monitoring breathing patterns and may find use as a sleep sensor in a hospital sleep lab. The system described therein utilizes a plastic film exhibiting both pyroelectric and piezoelectric properties, such as a polyvinylidene fluoride (PVDF) film. The film transducer has a conductive electrode on opposed major surfaces thereof and electrical wires connect the electrodes to an electronics module that is designed to separate the transducer output into two separate channels, one being for temperature and the other being for sound or vibration.
As is explained in copending application Ser. No. 09/416,660, filed Oct. 12, 1999, assigned to the assignee of the present invention and which application is hereby incorporated by reference, testing performed by the assignee of the present invention has shown that the signal proportional to temperature variation greatly exceeds that due to noise or vibration by a factor of about 50:1. The temperature-related signal also and is rich in frequency components in the 20 Hz to 50 Hz range during normal exhalation. These properties of earlier sensor designs have caused a difficulty in preventing the thermal component of the transducer from crossing over into the sound channel, often leading to the occurrence of false positives when both sound signals and temperature signals were being simultaneously monitored. The aforereferenced ""660 application describes a solution to the problem. By increasing the thermal mass of the sensor, the effective rise-time of the thermal signal can be greatly reduced, while not attenuating the signal amplitude occasioned by sound/vibration of the sensor below a detectable level.
As is further described in the aforereferenced ""660 application, the thermal mass of the PVDF transducer can be tailored by affixing a layer of plastic foam material onto the transducer film. The plastic foam material is made coextensive with the area of the PVDF film, i.e., it covered the entire surface of the PVDF film.
While the invention described in the aforereferenced ""660 application resulted in a significant improvement in the performance of the sensor in terms of being able to tailor the rise-time of the thermal signal in relation to the thickness of the foam plastic material applied, it did result in some loss of sensitivity of the transducer to sound/vibration signals.
It is, therefore, a principal object of the present invention to provide an improved pyro/piezo PVDF film transducer having a slower rise-time of the thermal signal to the point where it no longer overlaps with the predominant frequency components of the piezo or sound signal, but where the vibration/sound signal is not unduly attenuated.
In accordance with the present invention, control over the thermal mass of the transducer described in the aforereferenced Stasz ""875 patent is achieved by affixing a layer of plastic foam material unto the transducer film on at least one surface thereof but where the foam layer need not be coextensive with the PVDF film layer, thus leaving a portion thereof undamped by the plastic foam material. Thus, in accordance with the preferred embodiment of the present invention, the combination thermal and vibratory sensor comprises a plastic film exhibiting pyroelectric and piezoelectric properties and has a predetermined shape configuration with first and second major surfaces. A thin layer of conductive material is disposed on and is coextensive with the first and second major surfaces. A pair of elongated conductors are individually attached at one end thereof to the conductive material on the first and second major surfaces. A layer of a material for changing the thermal mass of the sensor is adhered to the layer of conductive material on at least one of the first and second major surfaces so as to overlay only a predetermined portion of the one major surface while leaving the remainder of that one major surface free of the layer of material. In use as a sleep sensor, the transducer of the present invention may be adhered to the upper lip of a person such that inspiratory and expiratory air flow, via the nasal passages, impinges primarily on the portion of the sensor to which the plastic foam material is adhered while the remainder portion of the transducer that is free of foam material either extends beyond the lips and over the mouth or engages the base of the nasal septum. Noise and vibration which may be caused by a person""s snoring remains undamped, allowing the film to output a more robust piezoelectric signal.